Studies on the specificity of benzo(a)pyrene oxygenation at carbon positions 1,3,5,7 and 9 and 3-MC-, BNF-, or Pb-induced microsomes prepared from liver, adrenal, and ovary suggest that the induction of microsomal cytochrome P-450s associated with the oxygenation of BP may be different between tissues of the same animal. In 3-MC- or BNF-induced hepatic microsomes, an increase in the rate of formation of the oxygenated metabolites of BP was detected at carbon positions 1 and 5, whereas in Pb-induced microsomes, an increase occurred only at C-1 and 5. In 3-MC-induced adrenal microsomes, an increase in the rate of benzo(a)pyrene oxygenation occurred at all carbon positions, whereas in 3-MC-induced ovarian microsomes an increase occurred at carbon positions 1,3,5 and 7 but not at carbon position 9. A comparison of the total amount of BP oxygenated metabolites formed at carbon positions 7 plus 9 to that formed at carbons 1 plus 3 plus 5 showed that purified cytochrome P-450 LM4 is 11 times more active than LM2 in the oxygenation of BP at the C-7 and 9- than at the C-1,3, and 5-positions.